PyQt: Replace bsd.pyqt.mk with Uses/pyqt.mk.
In preparation for landing PyQt5 ports, generalize devel/py-qt4's
bsd.pyqt.mk and make it a proper file in Uses/.
Ports wishing to depend on PyQt4 ports can now do the following:
USES= pyqt:4
USE_PYQT= foo bar_build baz_run
Other changes include the renaming of the PYQT4_DIST variable to PYQT_DIST
and the introduction of the PYQT_SIPDIR plist substitution variable. The
rest of the contents of Uses/pyqt.mk are pretty much identical to what we
had in bsd.pyqt.mk with additional processing of USE_PYQT.
Even though this patch touches files in many different ports, the goal is